In recent years, customer demand has increased on products having high sealing properties including water-proof performance and air-tightness performance in housings for electronic products such as mobile phones, digital cameras and video cameras, home electric appliances such as refrigerators, air conditioners and washing machines, and precision apparatuses such as cameras and watches. In mobile phones, for example, although hard resins such as nylon/GF have been used conventionally, in recent years, more inexpensive general-purpose resins such as ABS or ABS/PC are used. However, ABS, ABS/PC and the like are flexible and are likely to be deformed, and therefore conventional sealing materials have such a problem that satisfactory sealing properties cannot be achieved. Thus, sealing materials having higher flexibility and tensile strength than those of conventional sealing materials have been demanded.
As the conventional sealing materials, a hot-melt adhesive agent, a one-liquid type urethane-based reactive hot-melt adhesive agent, a thermoplastic hot-melt sealing material, a moisture-curable hot-melt sealing material, a heat-curable sealing material, a molding rubber sealing material (gasket), a two-liquid type urethane-based foamable sealing material and the like are known. However, all of these materials have problems of insufficient heat resistance, water-proof performance and the like, poor workability due to the requirement of preventing the inclusion of moisture or the like, thermal damage to devices and housings resulting from heating, poor workability due to the requirement of using a mold upon production and the like. Therefore, particularly when a flexible general-purpose resin is used for housings for electronic products, home electric appliances, precision apparatuses and so on, a satisfactory level of performance cannot be achieved.
A photocurable sealing material enables the production of a sealing material on a housing without using any mold, and therefore has excellent workability compared with those of conventional sealing materials. As the photocurable sealing material, there are two major materials, i.e., a photocurable sealing material for which radical polymerization of an acryloyl group is employed and an ene-thiol-type photocurable sealing material for which an ene-thiol reaction is employed. The photocurable sealing material for which radical polymerization of an acryloyl group is employed is disclosed in, for example, Patent Documents 1 and 2, which is produced by curing a composition including a urethane acrylate oligomer, an acrylate monomer, and a photopolymerization initiator (and a photo-sensitizer) with light or the like. However, the photocurable sealing materials disclosed in Patent Documents 1 and 2 do not have satisfactory levels of flexibility and tensile strength required for housings for electronic products, home electric appliances, precision apparatuses and so on.
The ene-thiol-type photocurable sealing material is disclosed in, for example, Patent Documents 3 to 6. In Patent Document 3, a photocurable resin composition containing a polyene, a polythiol, and a compound having a bromine-substituted aromatic ring is disclosed, and it is described that a cured product of the composition has a high refractive index, excellent adhesion strength and surface curability and the refractive index of the cured product can be controlled with high accuracy. In Patent Document 4, an ene-thiol-type photocurable resin composition containing a polyene compound and a (poly)thiol monomer including a polymercaptocarboxylic acid amide compound is disclosed, and it is described that the polymerization of the composition is not inhibited by oxygen, the composition can be cured within a short time and has a small volumetric contraction, the amount of a photo-initiator to be used in the composition can be reduced, and the moisture resistance of the composition can be improved greatly. In Patent Document 5, a curable composition containing a thiol compound having two or more groups of a specific structure and a urethane compound containing an ethylenically unsaturated double bond of a specific structure is disclosed, and it is described that a cured product of the composition has high adhesion to a base, high reactivity, high transparency, high heat resistance and high hardness. In Patent Document 6, a photocurable resin composition that contains polybutadiene (meth)acrylate, a polythiol compound and a photo-radical initiator and can be cured upon the irradiation by means of an ultraviolet ray LED is disclosed, and it is described that the composition can be cured rapidly and has excellent surface curability, excellent moisture resistance and excellent air-tightness performance. However, any of Patent Documents 3 to 6 do not mention in any way about flexibility, tensile strength and so on required for housings for electronic products, house electric appliances, precision apparatuses and the like.
A photocurable sealing material that can be cured by carrying out both the radical polymerization reaction of an acryloyl group and the ene-thiol reaction simultaneously and competitively is disclosed in Patent Documents 7 and 8. In Patent Documents 7 and 8, a material for gaskets is disclosed which includes (A) an oligomer having (meth)acryloyl group(s), (B) a (meth)acrylate monomer and (C) a polythiol compound, wherein the functional group ratio of the number of acryloyl groups to the number of mercapto groups is 100:0.1 to 100:5n, wherein n is the number of mercapto groups per molecule of the polythiol compound. In these Patent Documents, it is described that the material for gaskets can be improved in breaking elongation and can suppress the occurrence of cracking, breaking or the like. All polythiol compounds that are used specifically in Examples in Patent Document 7 are compounds each having a primary thiol group. However, all of cured products of the materials for gaskets obtained in Examples are insufficient with respect to any one of flexibility, breaking elongation (tensile strength) and compression recovery rate and there can be found no material that has all of the functions usable in housings for electronic products, house electric appliances, precision apparatuses or the like in Examples. Further, when it was tried to follow the examination of the sealing materials shown in Table 2 in Patent Document 7, it was found that the energy-ray-curable oligomer A (“Litetac PUA-KH32M”; produced by Kyoeisha Chemical Co., Ltd.) was no longer in production. Then, the oligomers and the monomers contained in the sealing materials were mixed properly and the examination was followed. As a result, as shown in Comparative Examples in the present description, only extremely hard sealing materials each having a hardness value of 50 or more were produced, and these sealing materials are not suitable as sealing materials for use in housings.